CLADDING TECHNOLOGY SHANXI CO., LTDResidual Stress Analysis in Strip Cladding Overlay Layer of Nuclear Island Main Equipment
Literature Overview
This study by Wu Yidang, Yang Zhipeng, Liu Mingyu, and Zhang Bin from CGN Engineering Co., Ltd. was published in the Journal of Nanchang Hangkong University (Natural Science Edition) in 2016. The research investigates residual stress distributions in strip cladding overlay layers applied to nuclear island main equipment, addressing a critical safety concern in nuclear power plant construction and maintenance.
Technical Background and Significance
Nuclear island main equipment — including reactor pressure vessels, steam generators, primary coolant loops, and safety injection systems — must operate under extreme conditions of temperature, pressure, and irradiation for decades. Strip cladding is widely used to provide corrosion and erosion resistance to these components while maintaining the structural integrity of the carbon or low-alloy steel base material. The residual stresses introduced during cladding welding can significantly affect the long-term performance and safety of nuclear equipment, making their characterization and control essential for regulatory compliance and operational reliability.
Residual Stress Generation Mechanisms
Residual stresses in strip cladding overlay layers arise from several sources:
- Thermal contraction — Differential cooling between the overlay layer and the base metal creates tensile stresses in the overlay and compressive stresses in the substrate.
- Phase transformations — Martensitic transformation in high-carbon overlay materials generates volume expansion that partially counteracts thermal contraction stresses.
- Plastic deformation — Localized plastic flow during welding creates complex stress patterns that depend on welding sequence and geometry.
- Constraint effects — The geometric constraints of the component and the welding sequence influence stress distribution.
Experimental Methodology
Test Specimen Configuration
| Parameter | Specification |
|---|---|
| Base plate material | 16MnR (Chinese standard, equivalent to ASTM A516 Gr.70) |
| Base plate thickness | 50 mm |
| Cladding material | 304 stainless steel strip |
| Strip thickness | 3 mm |
| Number of overlay passes | 3-5 |
| Welding process | Strip cladding (electroslag welding variant) |
| Test coupon size | 300 × 100 × 50 mm |
Residual Stress Measurement Techniques
| Technique | Spatial Resolution | Penetration Depth | Accuracy | Application |
|---|---|---|---|---|
| Hole drilling (strain gauge) | Point measurement | 0-2 mm | ±15 MPa | Surface stresses |
| X-ray diffraction (sin²ψ) | Point measurement | 0-0.5 mm | ±10 MPa | Near-surface stresses |
| Neutron diffraction | Volume measurement | 0-50 mm | ±5 MPa | Through-thickness stresses |
| Ultrasonic method | Volume measurement | 0-100 mm | ±20 MPa | Large area mapping |
| Contour method | Cross-section | Entire section | ±25 MPa | Full stress profile |
Residual Stress Distribution Results
Through-Thickness Stress Profile
| Depth from Surface (mm) | Longitudinal Stress (MPa) | Transverse Stress (MPa) | Notes |
|---|---|---|---|
| 0-1 (surface) | -80 to -150 (compressive) | -60 to -120 (compressive) | Surface compressive stresses beneficial |
| 1-3 (overlay layer) | +50 to +180 (tensile) | +40 to +150 (tensile) | Peak tensile stresses in overlay |
| 3-5 (interface) | +150 to +250 (tensile) | +120 to +200 (tensile) | Maximum tensile stresses at interface |
| 5-15 (base metal) | -30 to -80 (compressive) | -20 to -60 (compressive) | Compressive stresses in substrate |
| >15 (deep base) | -10 to -30 (compressive) | -5 to -20 (compressive) | Stress relaxation with depth |
Stress Distribution Along Weld Length
| Position | Longitudinal Stress (MPa) | Transverse Stress (MPa) | Notes |
|---|---|---|---|
| Weld start | +200 to +300 (tensile) | +150 to +250 (tensile) | High stresses due to constraint |
| Weld middle | +100 to +200 (tensile) | +80 to +180 (tensile) | Moderate stresses |
| Weld end | +150 to +280 (tensile) | +120 to +220 (tensile) | High stresses due to constraint |
Factors Influencing Residual Stress
Welding Parameter Effects
| Parameter | Effect on Residual Stress | Recommended Control |
|---|---|---|
| Heat input | Higher heat input → lower peak stresses | Optimize for process requirements |
| Travel speed | Faster speed → higher stresses | Moderate speeds preferred |
| Arc force | Higher force → deeper penetration → lower stresses | Adequate penetration required |
| Electrode composition | Higher alloy content → phase transformation effects | Match to application |
| Preheat temperature | Higher preheat → lower stresses | 150-250°C recommended |
| Interpass temperature | Lower interpass → higher stresses | Maintain >150°C |
Welding Sequence Effects
The welding sequence significantly influences residual stress distribution:
- Single-direction welding — Creates asymmetric stress patterns with high stresses at weld start and end.
- Alternating direction welding — Reduces net longitudinal stresses through self-balancing.
- Step-back welding — Distributes stresses more evenly along weld length.
- Multi-pass with stress relief — Each pass partially relieves stresses from previous passes.
Stress Relief Methods
Post-Weld Heat Treatment (PWHT)
| PWHT Condition | Stress Reduction | Hardness Change | Notes |
|---|---|---|---|
| 550°C × 2h | 60-70% | -10 to -20 HV | Effective stress relief |
| 600°C × 2h | 70-80% | -15 to -25 HV | Good balance of stress relief and properties |
| 650°C × 2h | 80-90% | -20 to -30 HV | May affect overlay properties |
| 700°C × 2h | 85-95% | -25 to -40 HV | Risk of softening overlay |
Mechanical Stress Relief
- Shot peening — Introduces surface compressive stresses of 200-400 MPa, effective for surface crack prevention.
- Vibration stress relief — Reduces residual stresses by 40-60% through controlled vibration.
- Rolling — Similar to shot peening, introduces surface compressive stresses.
Regulatory Requirements and Standards
| Standard | Requirement | Notes |
|---|---|---|
| GB/T 150 | PWHT required for cladding thickness > 6 mm | Mandatory for nuclear equipment |
| NB/T 47002 | Residual stress measurement and evaluation | Specific to nuclear equipment |
| ASME VIII Div.1 | PWHT per UG-120 for cladding | International standard |
| RCC-M | Stress measurement per Appendix CC | French nuclear standard |
| HAF 0300 | Chinese nuclear safety standard | Regulatory requirement |
Engineering Practice Considerations
For nuclear island equipment with strip cladding, the following practices are recommended:
- Pre-weld planning — Welding sequence design to minimize residual stresses through strategic pass arrangement.
- In-process monitoring — Real-time monitoring of welding parameters to ensure consistent heat input and stress levels.
- Post-weld stress measurement — Mandatory residual stress measurement at critical locations before and after PWHT.
- PWHT optimization — Heat treatment parameters selected to achieve adequate stress relief without compromising overlay properties.
- Quality documentation — Complete records of stress measurements, PWHT parameters, and post-PWHT verification for regulatory review.
Study Insights and Reflections
This research addresses a critical aspect of nuclear equipment fabrication that directly impacts safety and regulatory compliance. The detailed characterization of residual stress distributions provides engineers with the data needed to predict long-term behavior and design appropriate stress relief strategies.
The findings highlight the complexity of residual stress management in nuclear equipment. While post-weld heat treatment is effective for stress relief, it must be carefully controlled to avoid adverse effects on overlay properties, such as softening of martensitic overlays or sensitization of austenitic stainless steels. The balance between stress relief and property retention requires careful optimization for each specific application.
From a regulatory perspective, this work supports the development of more rational acceptance criteria for residual stresses in nuclear equipment. Rather than relying solely on PWHT as a stress relief method, engineers can now consider alternative approaches such as optimized welding sequences, mechanical stress relief, or even acceptance of residual stresses within defined limits based on their influence on fatigue and creep performance.
The research also underscores the importance of through-thickness stress characterization. Surface stress measurements alone are insufficient for evaluating the long-term performance of cladding, as the maximum tensile stresses often occur at the overlay-substrate interface, where they can drive interfacial cracking during thermal cycling or pressure loading.